In honor of Britney's big day, we took a trip to the ET vault to find our first interview with the megastar.
Just 17 at the time of the interview, Brit chats about her road to stardom, crediting her time on the Mickey Mouse Club as inspiration to pursue a career in music.
"After that show, that's when I realized I had a major love for music," Britney told ET in 1999, just months after "…Baby One More Time" hit the charts. "I had so much experience with acting and singing and dancing. It was a great experience for a lot of people. It really motivated me to go forth and go for what I wanted."
The birthday girl added that the show was clearly a vehicle for several of her peers, too, recalling the exact moment when she realized how many stars emerged for the Disney series.
"I was at a Teen People party like six months ago," she said, "there was Keri Russell, people from *NSYNC, Christina Aguilera... it was like a big Mickey Mouse Club reunion!"
Britney also went out of her way to explain that the spotlight didn't alter the Kentwood, La. raised starlet.
"When I go on stage I'm a different person, like I totally explode and I have attitude," she shared. "But, when I come off the stage, I'm just like this entertainment business has not affected me really at all. I'm still the same Britney, totally."
Though in later years, Britney publicly struggled with depression, custody battles over her two kids and control over her own career, as a young star the singer emphasized how unaffected she was by fame, adding, "I always do normal things, you have to do normal things so you'll stay sane which is you know, what I do all the time," she explained.
"On your days off, go out with the dancers and just go see a movie or just go shopping... You need to go out and do stuff, like, that you know is really, really important. I think coming from a really small town has really grounded me, you know? And the way I was raised and the way I was brought up you know has a lot to do with that.”
Today, Britney has sold more than 70 million albums and is holding down a successful residency at Planet Hollywood in Las Vegas, estimated to bring in $29 million for just 72 shows in two years, proving she's still one of the most sought-after performers.